The overall goals of the proposed work are: (i) to further establish the use of retrovirus-mediated gene transfer as a means for introducing genes into hematopietic cells, and as an approach for studying the properties of hematopietic stem cells, and (ii) to generate new information bearing on the feasibility of gene transfer as a therapy for human diseases affecting the hematopoietic system. The proposed experiments in the mouse focus predominantly on the use of gene transfer to study the processes of hematopoietic stem cell proliferation and differentiation, and to define the parameters that govern the expression of genes introduced into mature hematopoietic cell types via stem cell infection. These studies will involve the extensive characterization of hematopoietic tissue derived from long term bone marrow transplant recipients previously engrafted with retrovirus transduced stem cells. Assessment of the developmental potential of general properties of hematopoietic stem cells will rely on the use of recombinant proviruses introduced into stem cell DNA as unique clonal markers. Studies are also proposed to examine the transduction of human hematopoietic stem cells, and to establish a framework for studying the expression of genes introduced into those cells.